With increased interest in femtocells or Access Point Base Stations (APBS), i.e., smaller scale cellular base stations typically designed for use in residential or small business environments, there has been growing interest in reducing the cost associated with base station components, while maintaining or improving their performance. Cost is of a particular concern, as residential or small business users typically cannot afford the expense of conventional full base station implementations.
Femtocells typically connect to a service provider's network via broadband backhaul connections. A femtocell allows service providers to extend service coverage indoors, which is particularly useful where access would otherwise be limited or unavailable. The femtocell incorporates the functionality of a typical base station but extends it to allow a simpler, self contained deployment; for example, a Universal Mobile Telephone System (UMTS) femtocell may contain a base transceiver station (Node B), a Radio Network Controller (RNC) and a General Packet Radio Service Support Node (GSN) with Ethernet for backhaul. Femtocells are not limited to UMTS, the concept is applicable to all mobile radio systems, including GSM, CDMA-2000, TD-SCDMA and WiMAX solutions.
For a user, the attractions of a femtocell are improvements to both coverage and capacity, especially indoors. There may also be opportunity for new services and reduced cost. The cellular operator also benefits from the improved capacity and coverage but also can potentially reduce both capital expenditure and operating expense. However, due to timing synchronization requirements, such as holdover time, i.e., the time over which the femtocell stays synchronized with the main network without re-synchronizing, conventional femtocell implementations have utilized expensive oven controlled crystal oscillators (OCXOs), such as single oven oscillators costing in excess of $50, or double oven oscillator technology costing $300 or more.
Reductions in timing module component costs without significantly impacting performance would be beneficial to not only femtocell applications, but could potentially benefit any base station application.
The use of femtocells or other access point base stations indoors has conventionally meant that in applications in which the femtocell or other access point base station must be able to receive Global Navigation Satellite System (GNSS) signals, an antenna that is able to receive the GNSS signals must typically be located externally to the building in which the femtocell or base station is located, as direct reception of GNSS signals is often poor in indoor environments.